Then you have the stock ROM. "Stock" just means "what it comes with".
This means that you probably don't have init.d support. I'm pretty sure that Link2SD will have added an init script (look in /system/etc/init.d for something called 10apps2sd, or something similar). If the phone doesn't actually run that on boot then it's likely to break your Link2SD's app moving at some point. It's possible that your play store error 24 is related to that, especially if these are apps that you are trying to update rather than things you've never installed before: that error typically happens when there are app data left behind from an app that is not installed, so if you've moved apps and then the links are broken that could well produce this effect.
There are some XDA threads about enabling init.d support in stock ROMs.
This one still has an active discussion (I've only glanced at the first post, and as I don't know what device you have I obviously cannot guarantee compatibility, but it would seem worth a go). Another solution is
here, which I think should work too.
If your "just rooting" included installing a custom recovery, then do take a backup using it before messing with the system in any way (I'd personally include "before installing Link2SD" in that). If it doesn't, finding a custom recovery for your phone is something I'd recommend. I have a low opinion of rooting methods which just root the ROM without installing a custom recovery, because they give you more ways to mess up without providing the tools to back up and easily fix problems. Of course not all phones have custom recoveries built for them, in which case there's not much more you can do - you really must not install a recovery that was built for a different phone!